218 Chapter 3 Graphing Linear Equations in Two Variables 78. The store “CDs R US” sells all compact discs for $13.99.The following equation represents the revenue, y, (in dollars) generated by selling x CDs. y 13.99x 1x 02 a. Find y when x 13. b. Find xwhen . c. Write the ordered pairs from parts (a) and (b), and interpret their meaning in the context of the problem. d. Graph the ordered pairs and the line defined by the points. y 300 50 79. The value of a car depreciates once it is driven off of the dealer’s lot. For a Hyundai Accent, the value of the car is given by the equation where y is the value of the car in dollars x years after its purchase. (Source: Kelly Blue Book) a. Find y when b. Find xwhen . c. Write the ordered pairs from parts (a) and (b), and interpret their meaning in the context of the problem. x 1. y 9140 y 1025x 12,215 1x 02 y 279.80 250 200 0 2 4 6 8 10 12 14 16 18 20 Revenue ($) Number of Compact Discs 150 100 0 x Section 3.3 Slope of a Line and Rate of Change 1. Introduction to Slope The x- and y-intercepts represent the points where a line crosses the x- and y-axes. Another important feature of a line is its slope. Geometrically, the slope of a line measures the “steepness” of the line. For example, two hiking trails are depicted by the lines in Figure 3-14. Park Trail Mt. Dora Trail Figure 3-14 By visual inspection, Mt. Dora Trail is “steeper” than Park Trail.To measure the slope of a line quantitatively, consider two points on the line. The slope of the line is the ratio of the vertical change (change in y) between the two points and the horizontal change (change in x). As a memory device, we might think of the slope of a line as “rise over run.” See Figure 3-15. Slope change in y change in x rise run Concepts 1. Introduction to Slope 2. Slope Formula 3. Parallel and Perpendicular Lines 4. Applications of Slope: Rate of Change Change in x (run) Figure 3-15 Change in y (rise) Expanding Your Skills
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